1. Field of the Invention
This invention relates to a calculating rule useful for making eyeglasses meeting the eyes wearing the eyeglasses.
2. Description of the Prior Art
Making good eyeglasses requires for the maker to master meanings of power of the ametropia, accommodation, near-point, far-point, visual range, and addition, and the relationships thereof, and the calculation of obtaining any value from other values thereof, and to master the calculation for obtaining a resultant prism-diopter and base-direction from two prisms, each of which is characterized by the respective prism-diopter and base-direction, because heterophoria is ordinally represented by two prisms which have a respective prism-diopter and a respective base-direction, the two base-directions of the two prisms being any pair of orthogonal upward, downward, inward, and outward directions.
Mastering said businesses is not so easy for every maker. However, there has been heretofore no good means for helping master said businesses, except for the calculating rule disclosed by Japanese patent publication No. 7839/1978.
The calculating rule comprises a rectangular fixed rule and a rectangular slide rule; the fixed rule having two scales (A) and (D) that are apart from each other, the upper scale (A) being a scale of "accommodation" in dioptric unit and the lower scale (D) being a scale of "near-point distance" in centimeter unit, and the slide rule that slides between the upper scale (A) and the lower scale (D) having a scale (B) of "power of the ametropia" in dioptric unit that is also a scale of "addition" in dioptric unit, and a scale (C) of "far-point distance" in centimeter unit, and further, at the left end of the scale (C), an arrow mark for indicating the position of "near-point distance" on the scale (D).
The calculating rule enables to master the meanings of power of the ametropia, accommodation, near-point, far-point, visual range, and addition, and the relationships thereof, and to facilitate the calculation for obtaining any value from other values thereof.